Email sending errors (550, 554, authentication)
Updated on 08/10/2026
- 535 Authentication failed: wrong username (full address) or password, or SMTP authentication not ticked.
- 550 No such user: the recipient's address does not exist.
- 550 / 554 rejected, spam, blacklist: the message or sender is considered spam; check SPF/DKIM and send us the full error message.
- 552 Mailbox full: the recipient's mailbox is full.
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